Well we use the word in reference to the laws of physics that apply in the microscopic world
at the level of individual atoms, individual electrons, and elementary particles ... where
physics is qualitatively different than what we experience in our everyday lives.
Have you ever been to the beach and you see waves coming in?
Why do the waves have characteristic speed?
That's all classical physics.
Send a rocket to the moon....
There's no quantum physics in that
We're discussing right now the quantum world, which is at the very smallest scale... the
elements of reality down at that level are not described by what we're used to...
With the momentum of something... or the position...
...if you were to go and ask the same questions at the microscopic level, at the quantum level,
then things break down
So things are different there.
And that's a big part of why we're interested in this whole subject
Can we take this quantumness of the microscopic world and blow it up to larger and larger
sizes...
Manipulate them in a way which is highly controllable to get them to do what we want them to do
If you have a quantum system, that Quantum computer can do things; can perform tasks
that we couldn't hope to perform with ordinary digital computers.
You can't dream of doing it
So when we think about how quantum information is different from classical information now
a days, we're thinking about this intrinsic randomness... the uncertainty principle...
and also about entanglement-
Classical information we can express in terms of bits, you can take any amount of information
and write it as a string of bits.
And in the quantum case... we call them quantum bits, or Q bits, and they're different from
ordinary bits in some fundamental ways.
So a classical bit I can write a zero or a one on the table and everybody can look at
it the same way, but in a quantum bit there's more than one complimentary way to look at
it.
You can kind of picture it this way, you've got a box and you can put an object in the
box.
In the quantum case there is more than one way to open the box.
And you've got to make a choice,
If you open door number one, you'll never know what would have happened, it cannot be
known, what would have happened if you had opened door number two instead, those are
incompatible things.
That's something really new, compared to classical physics
That's what I mean when I say it's really intrinsic randomness.
It's not that there's some record somewhere, but you haven't looked at it yet...
There's no record.
It hasn't been decided yet ... whether it's going to be a zero or a one.
It's not until you make the observation and open the door that it becomes the value of
this bit, maybe it's a zero, maybe it's a one.
The randomness comes from the ability of quantum states to be in a superposition.... to be
both in the state of zero, of definite say spin up for some like particle, and also a
spin down state.
What if they were not just numbers, they were waves, then you can, like, put together two
of these waves and you can think of having half of a zero and half of a one, so whenever
you measure it, you will end up getting half of the time the zero and half of the time
the one.
So superposition is fundamental for that randomness.
If I flip a coin, I don't carry coins in my pocket anymore or I'd flip one, I flip a coin
and then I cover up the coin as soon as it lands you don't know whether it's a heads
or a tails, but it's either a heads or a tails we just don't know yet, that's not a superposition,
that's a probability distribution.
But if we have the most complete description of the system that is possible, compatible
with quantum physics, like when I prepared the state and when we open door number two
we don't know whether it was a zero or a one- that's superposition, because that's really
intrinsic randomness instead of probability associated with ignorance.
So, it takes two to tango in quantum entanglement...
You cannot just have a single Qbit and say, oh, it's entangled.
With what?
So you need two q bits, at least.
The reason these quantum correlations are different than classical correlations is because
we have these distinct incompatible ways of observing the system.
Classical systems can be correlated, we can flip two coins and they're either both heads
or their both tails, no big deal, but there's just one way to look at that coin.
Q bits are different, we have these two incompatible ways of looking at them, they have two ways
to be correlated, either the same or different for two different sets of doors, those are
all together four possibilities.
And that means the correlations are richer, they're more interesting, and the richness
of the correlations increases very markedly as you increase the number of parts or add
additional q bits.
You know, if you have just a few hundred q bits and you want to write down a complete
description of all those correlations among those q bits in terms of classical bits, you'd
have to write down more numbers than the number of atoms in the visible universe.
It's one of these things like...
I don't know if it was Faust that said it; that achieving perfection is, hard ..., but
remaining perfect, that's impossible.
A lot of what we're trying to do is create things that are stable, because you can have
the most exotic quantum state but unless you can probe it, measure it, evolve it in some
way that you want to steer it, it's going to be useless.
There's a central problem...
And that is, if you're running a quantum computation, if you're performing a sequence of operations,
you know, processing the quantum state...
That has to be very well concealed, not just from you and from Spiros and from me but from
the whole outside world.
Classically there'd be nothing wrong with looking at every time step what the state
of the computer is... that wouldn't prevent the computer from getting the right but quantumly
if we keep looking at the computer that will 'destroy' these delicate superpositions.
It's a secret computation, until the end when we're finally ready to get the result out,
we make a measurement and then its okay to tell everybody what the outcome is; to broadcast
it to your friends.
But we can't be looking at the computer while it's performing the computation
If we ask it afterwards, what did you just do when you factored that huge number, it
should say, "I don't remember."
Because there was no record left behind of what it was doing at intermediate stages of
the computation.
And this is what makes it so challenging.
Because we can't allow any leakage of information, from the computer, to the surroundings, that
would destroy the quantum computer
That's what we call decoherence, it's the big enemy.
There's a whole quantum world out there which is largely unexplored because it's only now,
within the last decade or so, that we are developing the technological capabilities
to scale up quantum systems to manipulate them, and we're not exactly sure what that's
going to lead to.
But we think it's exciting.
And I do get excited!